As electric equipment, rotating machines are classified as direct- and alternating-current machines. Among them, the alternating-current machines generate alternating-current power by receiving mechanical power or generate mechanical power by receiving alternating-current power, and they are mainly classified as induction and synchronous machines.
The induction machine, e.g., an induction motor, rotates by generating an induced torque on a rotor with a rotating magnetic field generated by applying alternating-current voltage to a stator winding. The induction motor is widely used because of, for example, its simplified structure, easy maintenance and low cost, but it has disadvantages in terms of efficiency and speed control.
The synchronous machine, e.g., a synchronous motor, rotates by a rotor, which includes an electric or permanent magnet, being attracted by a rotating magnetic field generated by applying alternating-current voltage to a stator winding. The synchronous motor is efficient but requires additional devices for start-up and pulling into synchronism.
Therefore, in recent years, there have been proposed superconductive rotating machines capable of synchronous rotation while having induction machine configurations (see, for example, Patent Documents 1 and 2).
The rotating machine described in Patent Document 1, as shown in, for example, FIG. 6 therein, includes a stator 60, a rotor 61 rotatably attached to the stator 60, a superconductive material 62 provided to the rotor 61, a magnetic field generator provided to the stator 60 to form a rotational magnetic field, a mechanism for trapping in the superconductive material 62 a magnetic field penetrating through the superconductive material 62, and a torque shield 64 disposed between the magnetic field generator and the superconductive material 62, which has such a skin depth and thickness as to keep the magnetic field intensity in the superconductive material 62 lower than a second critical magnetic field Hc2, and also has sufficient electrically conductive properties to produce enough torque to accelerate the rotor 61 to synchronous speed.
At start-up, the rotating machine described in Patent Document 1 is inductively rotated by an induced torque generated on the torque shield 64. Then, when a predetermined speed is reached, magnetic flux in the rotational magnetic field extends through the torque shield 64 into the superconductive material 62. Thereafter, when the superconductive material 62 is cooled to a critical temperature or lower and brought into superconductive state, the magnetic flux in the rotational magnetic field is trapped in the superconductive material 62, so that the rotating machine described in Patent Document 1 is synchronously rotated.
On the other hand, the electric motor described in Patent Document 2, as shown in, for example, FIGS. 3 and 4 therein, has a superconductive material 13 filling both hollow portions 10 of bars and grooves 11 and 12 in end rings 5 within a squirrel-cage winding made of a normally conductive material. Specifically, the electric motor described in Patent Document 2 has a structure in which a squirrel-cage winding made of a normally conductive material is provided together with a closed circuit acting as a field winding made of a superconductive material.
The electric motor described in Patent Document 2 can be started as an ordinary squirrel-cage induction machine with satisfactory start-up characteristics by starting it up in the atmosphere at room temperature. Also, upon completion of acceleration after the start-up, the squirrel-cage rotor is cooled to or below a critical temperature of the superconductive material to forma closed circuit of the superconductive material, so that the rotor is automatically pulled into synchronization, and thereafter can operate with extremely high efficiency as a synchronous motor under constant current.    Patent Document 1: Japanese National Phase PCT Laid-Open Publication No. 8-505515    Patent Document 2: Japanese Laid-Open Patent Publication No. 1-144346